i read what tileguybob had to say about tiling over a marble fireplace but did not understand how or where the "support" board goes on. can someone please clarify that? and can you lay tile over cement?

I haven't seen the post from Bob that you're talking about, but I'd imagine he was talking about what's known as a "ledger board" which goes under the second course of tile, so you can do the whole installation, leaving the bottom row for last in case it has to be cut in at all.

Like I said, I didn't see the post, but I'd imagine that's what you're talking about. As for laying over concrete, so long as there's no cracks, absolutely.

use the ledger board over the horizontal row over the fireplace so the tile doesn't sag
wait for thinset to cure before removing ledger
then you can tile up both sides to that bridged row

Matt
Cupan Custom Tile and Paint
tile contractor